In a conventional form, a liquid crystal display (LCD) device consists of a liquid crystal layer with opposite sides, a set of electrodes (electrical conductors such as indium-tin-oxide, ITO) on either side of the liquid crystal layer and an alignment or orientation polymer layer between each set of electrodes and the liquid crystal layer.
Alignment of the liquid crystal molecules occurs along a preferred direction, e.g. introduced by rubbing of an orientation layer, and at a certain angle with respect to the plane of the inside of the two substrates, usually glass plates or plastic sheets, which support the electrodes. This latter angle is referred to as the tilt angle as disclosed by T. Uchida and H. Seki in Liquid Crystals Applications and Uses, B. Bahadur, ed., World Scientific, London, Vol. 3, 1992, pp 1-63; G. Baur et al. in Physics Letters, Vol. 56A, No. 2, 1976, pp. 142-144; T. J. Scheffer and J. Nehring in J. Appl. Phys. Vol. 48, No. 5, 1977, pp. 1783-1792.
The set of transparent electrodes are patterned, e.g. by etching, in a layout compatible with the information to be displayed by the LCD.
The alignment layer is applied onto the two glass plates or plastic sheets covered with ITO substrates, by solution casting, e.g. spin coating, roller coating, dipping, spraying, printing and/or doctor blading, of the appropriate polymer or its precursor. After removal of the solvents and/or curing of the polymer layers, the substrates are usually rubbed or buffed in one direction with cloths. The rubbing process serves to establish a unique optical direction. After rubbing both plates or sheets, they are rotated by a certain angle around the axis perpendicular to the main plane in order to establish the so called twist angle in the liquid crystal. In Super Twisted Nematic (STN) applications for liquid crystal displays the twist angle of the liquid crystal has a value preferably between 160 degrees and 360 degrees as disclosed by C.M. Waters et al. in Proc. Soc. Inf. Displ., 3rd Intern. Display Conference, Kobe, Japan, New York, 1985; for STN-LCDs: Y. Kando, T. Nakagomi, S. Hasegawa, C. Mobara in German Offenlegungsschrift 3,503,259; for SBE-LCDs: T. J. Scheffer and J. Nehring in Appl. Phys. Lett. 45, 1984, p. 1021; for OMI-LCDs: M. Schadt and F. Lennhouts in Appl. Phys. Lett. 50, 1987, p. 236; for DST-LCDs: F. Jonas, R. Dhein in European Published Specification 0,236,842; for BW-STN-LCDs: K. Kawasaki et al. in SID 87 Digest, 391, 20.6).
In order to establish a cell, the plates or sheets are adhered together e.g. using organic adhesives to preserve a constant thickness of the space or gap between the surfaces which may be achieved by the use of suitable spacers in the frame adhesive and/or over the whole display area. The cell is filled with a mixture of liquid crystal materials and finally sealed using e.g. organic adhesives. At this stage, polarizing films are often attached to the outside glass or plastic surfaces by a lamination process. In addition to polarizing films other optically active layers are frequently applied, such as compensation layers, .lambda./4 plates and brightness enhancement films. Finally, electrical connections are made to both plates or sheets in a manner consistent with the electrical and display designs.
A general problem of LCDs which is especially severe in the case of STN LCDs is their insufficient viewing angle. An effective method to improve this parameter is given by introduction of domains with different alignment directions into each addressed pixel of a liquid crystal display device, leading to so called Multi-Domain Displays.
For the sake of simplicity the closer explanation of this type of display is given in a simple modification, the Double-Domain LC-display, also called Duel-Domain LCD as disclosed by T. Yamamoto, Y. Morizumi, M. Okamoto, Y. Tanaka, N. Fukuoka, H. Hatoh in J. SID, 4/2, 1996, pp. 83-87; Z. Ma, K. Ma, H. Jing, S. Wu, X. Zhu, Y. Liang, J. Guo, J. Yuan, X. Huang in SID 96 Digest, 1996, pp. 658-661; J. Li, E. S. Lee, H. Vithana, P. J. Bos in Jpn. J. Appl. Phys., Vol. 35, 1996, pp. 1446-1448. It is obvious that the following is valid for any larger number of domains.
Each of the substrate surfaces of a Double-Domain STN-cell, addressable through ITO electrodes, i.e. pixels are divided into two domains with alignment directions opposite to each other. The twist angle of each of the domains is in the same range used for standard STN-applications i.e. Single-Domain STN-LCDs (generally between 160 degrees and 360 degrees). Each of the domains in a Double-Domain LCD therefore can be regarded as a single cell displaying half of the given information with the same contrast but for mutually opposite viewing angles to the corresponding second domain.
Double-Domain STN-cells with perpendicular alignment directions of the domains have also been described, leading to an improved contrast for a different viewing angle.
The process serving to establish the two different domains is similar to conventional STN-fabrication processes where an alignment layer, such as a polyimide, is rubbed, as described above, to determine the alignment direction. In addition to this step, for Double-Domain STN LCDs a masking operation is performed e.g. by photolithography which as a result covers half of the pixel with a removable layer. This is followed by a second rubbing process. The second rubbing effects only the non-masked area and is performed in a predetermined direction differing from the first rubbing direction. After removing the mask with a solvent, an orientation layer with two areas of different alignment direction results. This process is carried out on both the top and the bottom surface of the cell, with rubbing directions altered correspondingly at least on one substrate, to achieve the twist angle of an STN-LCD. Upon assembly of the two substrates, the LCD is formed which has pixels with two domains in each pixel that mutually compensate each others dependence of the contrast on the viewing angle.
The use of rubbed polymer films, i.e. alignment direction and tilt angle controlling films, dominates the process technology used in the production of all categories of liquid crystal displays. Moreover, the tilt angle and its magnitude are most important in the various electro-optic display modes. The stability, legibility and reliability of the LCD are all related to the magnitude and stability of the tilt angle.
Polyimides are generally known as orientation layers in liquid-crystal displays and have proven successful due to their good optical and thermal properties. They are also employed, in particular, in STN-display elements.
Frame sealants used in the LCD are typically organic adhesives which are sealable by exposure to UV radiation or to elevated temperature. Commonly used temperature curable organic adhesives are set at a typical peak temperature of 150.degree. C. to 200.degree. C. for a curing time ranging from several minutes to several hours. The effect of alignment which is induced by the rubbing of the alignment layer has to withstand this temperature treatment as the sealant has to be cured after rubbing of the orientation layer.
In the production process of Multi-Domain LCDs the demand on the alignment layer is even higher than that of Single-Domain LCDs since an already rubbed layer with a certain alignment direction is subjected to a masking procedure and a second rubbing process with a different rubbing direction as described above. Thus, the alignment polymer layer and the alignment direction which directly effects the tilt angle being induced in a liquid crystal layer, need to be stable towards developing agents, such as hydroxides like tetramethylammoniumhydroxide, which are employed in the photolithographic process generating the mask. Furthermore, the polymer alignment layer and the alignment direction have to be stable towards contact with the mask and towards solvents, such as dialkylethers like diethylether or esters like butylacetate or ethylacetate which may be used in order to remove the mask from the alignment layer.
However, it is almost generally found in the production process of Multi-Domain LCD that the value of the tilt angle exhibit reductions of more than 10% compared to the tilt angle of any other domain of an observed pixel, after multiple rubbing, high curing temperatures and masking procedures have been applied to the alignment layer.
This difference in the tilt angle of the domains of a pixel can be explained by two major effects on the alignment layer, adding up during the manufacturing process of the Multi-Domain LCD. First, by rubbing an already rubbed alignment layer in a different direction the ability to orientate liquid crystals is usually weakened. Thus the tilt angle provided by such a double rubbed alignment layer decreases significantly, compared to the tilt angle obtained by the single rubbed alignment layer when using standard polyimides as orientation layers, under the same rubbing conditions and even under optimized rubbing conditions for the second rubbing.
Second, the value of the initial tilt angle decreases usually to a large extent by bringing an already rubbed alignment layer into contact with dialkylethers or esters and hydroxides during the masking procedure in the production of Multi-Domain LCDs. Usually in the Multi-Domain LCD production process one of the areas of the alignment layer establishing the domains of a pixel is rubbed once, therefore provides a high tilt angle and stays protected by the photolithographic mask till the end of the process, whereas the others are subjected 4; to the above conditions, exhibit a reduced tilt angle compared to this initial tilt angle and thus giving rise to the difference in tilt angle in the set of domains of a pixel after assembly of the cell.
In addition to these adverse effects it was also found that high temperatures which are employed in the curing process of temperature curable adhesives significantly decrease the value of the tilt angle provided by standard polyimide alignment layers.
Especially for Multi-Domain STN-applications, where larger tilt angles are required as disclosed by T. Scheffer in Appl. Phys. Left., 45, 1984, pp. 1021-1023, the reduction of the tilt angle occurring in some of the domains of a pixel, leads to a significant loss of contrast and to an enhanced viewing angle dependence. This is even more noticeable in the case when multiplexed cells, as disclosed by T. Scheffer, J. Nehring in Liquid Crystals Application and Uses, B. Bahadur, ed., World Scientific, London, Vol. 1, 1990, are envisaged.
It was found that a tilt angle reduction larger than 10% between two domains of the pixel of a cell is already sufficient to have an adverse effect on the optical performance of the cell.
Therefore a need exists for polyimides that maintain the large tilt angle used for STN-applications or at least do not exhibit reductions of the tilt angle of more than 10% compared to the tilt angle of any other domain of the pixel, after multiple rubbing, high curing temperatures and masking procedures have been applied to the alignment layer in the fabrication process of Multi-Domain LCDs.
Several fluorine containing polyimides are known to be suitable as alignment layers for Single-Domain STN LCDs which provide moderate to high tilt angles.
For example, Japanese Kokai Patent No. 63(1988)-259515, published Oct. 26, 1988, Japanese Kokai Patent No. 1(1989)-180518 and No. 1(1989)-180519, published Jul. 18, 1989 and Japanese Kokai Patent No. 2(1990)4225, published Jan. 9, 1990, disclose polyimides as alignment layers for liquid crystal displays containing perfluoroalkyl groups, comprising the compound of formula (I) as a component which is abbreviated in the following as BDAF ##STR1## Throughout the text of this application, above and below, Roman figures in parentheses are used as an abbreviation for a formula of that number.
Japanese Kokai Patent No. 62(1987)-127827, published Jun. 10, 1987 and Japanese Kokai Patent No. 62(1987)-87939, published on Apr. 22, 1987 disclose fluorinated alignment films for liquid crystals comprising compounds of formulae (I) (BDAF) and (II), which is abbreviated in the following as 6FDA, as components ##STR2## WO 9217439 describes the use of compounds of formulae (II) and (III) which is abbreviated in the following as Rf.sub.6 DAM ##STR3## U.S. Pat. No. 5,520,845 discloses compounds of formulae (IV) and (V) which are abbreviated in the following as Rf.sub.8 MPD and Rf.sub.10 MPD respectively, as components of LC alignment layers ##STR4## and U.S. Pat. No. 5,175,367 discloses a compound of formula (VI) which is abbreviated in the following as TFMOB, as an ingredient of polyimide films ##STR5##
Compounds of formulae (VII) and (VIII) which are abbreviated in the following as P3FDA and P6FDA respectively, are disclosed as components of polyimides in Macromolecules, Vol. 25, No. 13, 1992 ##STR6##
U.S. Pat. No. 5,344,916 discloses a compound of formula (IX) which is abbreviated in the following as TFMB as a component of polyimides used for birefringent films ##STR7##
European Patent Application EP 413194 discloses a compound of formula (X) as an ingredient of polyimides ##STR8##
European Patent Application EP 415447 discloses a Liquid-crystal device having a polyimide film as an alignment layer comprising a compound of formula (XI) ##STR9##
U.S. Pat. No. 5,395,918 discloses polymides comprising compounds of the formula (XII), wherein Z may be a halogenated alkyl group of 1 to 3 carbon atoms ##STR10##
U.S. Pat. No. 5,051,520 discloses compounds of formulae (XIII) and (XIV) as components of polyimides. (XIII) and (XIV) are abbreviated in the following as 6FCDA and 3FCDA respectively ##STR11##
Finally, U.S. Pat. No. 5,186,985 discloses polyimide alignment layers having pendant fluorinated chains. These alignment layers are capable to provide tilt angles of 5 or more degrees and may comprise a compound of formula (XV) which is abbreviated in the following as Rf.sub.b MPD. ##STR12##
However, none of these prior art documents, disclose a polyimide alignment film providing a high tilt angle suitable for STN LCDs that maintains the tilt angle with no or only a small reduction during the conditions of the fabrication process of a Multi-Domain LCD which includes high curing temperatures and multiple rubbing and masking procedures as described above.